The invention relates to a method for processing the black liquor of a pulp mill to recover the chemicals and energy therein. The invention further relates to an apparatus for processing the black liquor of a pulp mill to recover the chemicals and energy therein.
In a pulp process, wood raw material, such as chips, is processed using heat and chemicals by cooking it in a chemical solution that contains lye among other things. This is called pulping. The aim of the process is to remove fibre-binding lignin. The cooking chemicals used in sulphate cooking are a mixture of sodium hydroxide (NaOH) and sodium sulphide (Na2S). In soda cooking, the cooking chemical is sodium hydroxide (NaOH). In carbonate cooking, the cooking chemical is sodium carbonate (NaCO3). After cooking, the fibre pulp detached from the wood material is separated from the pulping liquor in which the various wood material components, such as lignin and inorganic matter, dissolved during cooking remain. This chemical mixture, that is, black liquor, separated after cooking is then evaporated in an evaporation plant so as to produce a burnable material that contains as little water as possible. This material obtained from the final step of the evaporation plant and conventionally fed into a recovery boiler to be burned may have a dry matter content of up to 85%.
Traditionally, black liquor is burned in a recovery boiler, whereby vapour and electricity is produced for use as energy for the mill and possibly for sale. The inorganic part of black liquor is removed from the recovery boiler as salt in the liquid state; this produces heat, with which water is heated and vaporised to produce energy, and salt in the liquid state, from which cooking chemicals are re-made. This is disclosed in Finnish patents 82494 and 91290, for example.
Black liquor gasification has been tried as a replacement for the recovery boiler, but a commercially competitive solution is yet to be achieved in practice.
WO publication 2004/005610 discloses a solution, wherein black liquor is pyrolyzed and the coke from the pyrolysis is gasified. However, this process is in practice difficult and requires a separate and expensive gasification apparatus.